COMA-FREE ALIGNMENT OF A HIGH-RESOLUTION ELECTRON-MICROSCOPE WITH 3-FOLD ASTIGMATISM

Coma-free alignment which eliminates coma aberration is important for high-resolution electron microscopy. It is found that the coma-free alignment routine based on the astigmatism at diametrical tilts is ineffective for microscopes with a small spherical aberration coefficient, such as the Akashi (now Topcon) EM-002A. Therefore, new algorithms for coma-free alignment, which take into account three-fold astigmatism, are devised on the basis of beam-tilt-induced astigmatism and/or focus change. Their performances are evaluated on the Akashi EM-OMA (120 kV) using high-quality carbon images obtained through a slow-scan CCD camera. The routine based on the beam-tilt-induced focus change gives a stable estimate of the coma-free direction with an estimation error typically better than 0.2 mrad. The developed routines can also estimate the magnitude of three-fold astigmatism. It is found that wave aberration due to three-fold astigmatism is relatively small and may be neglected for conventional high-resolution electron microscopy. However, three-fold astigmatism may introduce an appreciable error for the coma-free estimation and consequently will affect high-resolution electron microscopy.